From the existing art, a large number of sensor elements and methods are known for acquiring at least one property of a measurement gas in a measurement gas compartment. In principle, these can be any physical and/or chemical properties of the measurement gas, and one or more properties can be acquired.
For example, such sensor elements can be fashioned as so-called lambda sensors, as known for example from Konrad Reif (pub.): Sensoren im Kraftfahrzeug, 1st edition 2010, pp. 160-165. With broadband lambda sensors, in particular planar broadband lambda sensors, for example the oxygen concentration in the exhaust gas can be determined in a large range, and in this way the air/fuel ratio in the combustion chamber can be inferred. The air index λ describes this air/fuel ratio.
From the existing art, in particular ceramic sensor elements are known that are based on the use of electrolytic properties of particular solid bodies, i.e., on ion-conducting properties of these solid bodies. These solid bodies can for example be ceramic solid electrolytes, such as zirconium dioxide (ZrO2), in particular yttrium-stabilized zirconium dioxide (YSZ), and scandium-doped zirconium dioxide (ScSZ), which can contain small additives of aluminum oxide (Al2O3) and/or silicon oxide (SiO2).
To optimize pollutant emissions and exhaust gas post-treatment, in modern internal combustion engines lambda sensors are used to determine the composition of the exhaust gas and to control the internal combustion engine. Lambda sensors determine the oxygen content of the exhaust gas, which is used to regulate the air/fuel mixture supplied to the internal combustion engine and thus the exhaust gas lambda before a catalytic converter. Here, via a lambda regulating circuit the air and fuel supply to the internal combustion engine is regulated in such a way that a composition of the exhaust gas is achieved that is optimal for the exhaust gas post-treatment by catalytic converters provided in the exhaust duct of the internal combustion engine. In spark-ignition engines, as a rule regulation takes place to a lambda value of 1, i.e., a stoichiometric ratio of air to fuel. In this way, the pollutant emission of the internal combustion engine can be minimized.
Various forms of lambda sensors are in use. A broadband lambda sensor, also referred to as a constant or linear lambda sensor, enables the measurement of the lambda value in the exhaust gas in a broad range around lambda=1. In this way, for example an internal combustion engine can also be regulated to a lean operation with an air excess. Through a linearization of the sensor characteristic curve, a constant lambda regulation before the catalytic converter is also possible using a lower-cost two-point lambda sensor, though in a limited lambda range. In such a two-point lambda sensor, also called a discrete-level sensor or Nernst sensor, the voltage-lambda characteristic curve has an abrupt dropoff at λ=1. It therefore essentially permits a distinction to be made between a rich exhaust gas (λ<1) during operation of the internal combustion engine with a fuel excess and lean exhaust gas (λ>1) during operation with an air excess, and permits regulation of the exhaust gas to a lambda value of 1.
A precondition for the constant lambda regulation with a two-point lambda sensor is that an unambiguous relation exists between the sensor voltage of the two-point lambda sensor and lambda. This relation must be present over the entire lifespan of the two-point lambda sensor, because otherwise the precision of the regulation is not adequate, and impermissibly high emissions can occur. This precondition is not met due to manufacturing tolerances and aging effects of the two-point lambda sensor. Instead, the actual sensor characteristic curve can be shifted relative to the reference sensor characteristic curve by a plurality of superposed effects.
Therefore, two-point lambda sensors before the catalytic converter are usually used with a two-point regulation. This has the disadvantage that in operating modes for which a lean or rich air/fuel mixture is required, for example for catalytic converter diagnosis or for component protection, the target lambda can be set only in pre-controlled fashion, but cannot be regulated.
Despite the advantages of methods known from the existing art for recognizing a voltage offset of a voltage-lambda characteristic curve, these methods still have room for potential improvement. Thus, DE 10 2012 211 687 A1 describes a method for recognizing a voltage offset of a voltage-lambda characteristic curve, with which shifts of the actual sensor characteristic curve relative to the reference characteristic curve can be recognized and compensated. In this way, a constant lambda regulation with a two-point lambda sensor before the catalytic converter is possible. For the recognition of temperature-caused characteristic curve shifts, this method uses a change in the air/fuel mixture based on a value pair that is to be checked on the reference voltage-lambda characteristic curve of the two-point lambda sensor up to lambda=1. From the change of the composition of the air/fuel mixture up to the reaching of lambda=1, the actual value of lambda before the change is inferred. This procedure presupposes that the change takes place with a clearly defined profile, such as for example a ramp shape having a clearly defined rise.
As a rule, this precondition is not met at all engine operating points. In particular given short injection times, the injection valve characteristic curve, which describes the functional dependence of the injected fuel quantity on the injection time, is as a rule wavy, and a reference injection valve characteristic curve, stored in the control device, takes this waviness into account.